I tried installing UnVersion using NuGet.

Then I modified unVersion.config to the following:

<?xml version="1.0"?>
<unVersionConfig>
    <add maxDays="30" maxCount="3" />
</unVersionConfig>

Then I save-and-published a Document with a lot of old versions (a year old). But the number of records in cmsContentVersion increased by one (just like before) instead of reducing. And I can still see the old versions in "Rollback".

EDIT: Solved! I had to modify web.config and reload the CMS for it to take effect.